A radiation-emitting chip of the type mentioned generally has a multilayer structure with an active radiation-generating layer, which is applied to a substrate. Radiation is coupled out at least partly through the substrate, the substrate being transparent to the radiation generated.
In this arrangement, the radiation efficiency is often greatly limited by total reflection at the substrate surface. This problem area occurs particularly in the case of substrates having a high refractive index, for example an SiC substrate, and a correspondingly small angle of total reflection.
This is especially the case with a substrate having a square or rectangular cross section, in which multiple successive total reflections limit the radiation efficiency. This is illustrated by way of example in FIG. 8, which shows a section through a radiation-transmissive substrate 20. If a radiation portion 1 propagating in the sectional plane impinges on an interface of the substrate 20, then it is at least partly coupled out if the angle θ1 of incidence is less than the angle αt of total reflection. Here and below, angle of incidence and angle of total reflection relate to the normal to the interface.
If the angle θ1 of incidence is greater than the angle αt of total reflection, as illustrated, only relevant radiation portion 1 is subjected to total reflection. The angle αt of total reflection thus defines a so-called coupling-out cone 3, illustrated in the section by the dashed boundaries 4a, 4b, the aperture angle of which amounts to 2αt. If the relevant radiation portion 1 impinges on the interface in such a way that it lies within the coupling-out cone 3, then it is at least partly coupled out, otherwise it is subjected to total reflection.
In the example illustrated the substrate 20 has a square cross section. The radiation portion 1 continuously impinges on the interfaces of the substrate 20 outside the coupling-out cone 3. Therefore, the radiation portion 1 circulates cyclically in the substrate 20 whilst undergoing multiple total reflection and is finally absorbed without being coupled out beforehand.